1. Field
The invention concerns a process for the production of water-soluble, naturally folded and secreted polypeptides after expression in prokaryotic cells by co-secretion of molecular chaperones.
2. Description
Protein synthesis in prokaryotic organisms, which is also called translation, takes place on the ribosomes in the cytoplasm. When recombinant DNA is expressed in prokaryotic host organisms, it is often desirable to secrete the recombinant gene product or protein that is obtained in this process from the cytoplasm through the inner bacterial membrane into the periplasmic space between the inner and outer membrane. Secreted proteins can then be released from the periplasm into the nutrient medium for example by an osmotic shock. A disadvantage of this process is that the secreted polypeptides often do not form the native, biologically active conformation (Hockney, TIBTECH 12 (1994) 456-463).
Recently molecular chaperones and folding catalysts such as peptidyl-prolyl-cis/trans-isomerases or protein disulfide isomerases (Glockshuber et al., EP-A 0 510 658) have been used to increase the yield of native recombinant protein when folded in vivo (Thomas et al., Appl. Biochem. Biotechnol. 66 (1997) 197-238). In some cases this has led to considerable improvements in the expression e.g. of ribulose bisphosphate carboxylase (RUBISCO; Goloubinoff et al., Nature 337 (1989) 44-47), human procollagenase (Lee and Olins, J. Biol. Chem. 267 (1992) 2849-2852) or neuronal nitrogen oxide synthase from rats (Roman et al., Proc. Natl. Acad. Sci. USA 92 (1995) 8428-8432). In these examples GroEL/ES or the DnaK system from E. coli was co-overexpressed in the cytosol.
The co-expression of chaperones has also been examined when recombinant proteins are secreted into the periplasm of E. coli. However, in this case only a cytosolic overexpression of chaperones was evaluated in order to optimize secretion into the periplasm (Perez-Perez et al., Biochem. Biophys. Res. Commun. 210 (1995) 524-529; Sato et al., Biochem. Biophys. Res. Commun. 202 (1994) 258-264; Berges et al., Appl. Environ. Microbiol. 62 (1996) 55-60). Previous attempts at co-secretion in E. coli have concerned folding-helper proteins such as e.g. protein disulfide isomerase (PDI; Glockshuber et al., EP-A 0 510 658), peptidyl-prolyl-cis/trans-isomerases, Dsb proteins (Knappik et al., Bio/Technology 11 (1993) 77-83; Qiu et al., Appl. Environm. Microbiol. 64 (1998) 4891-4896 and Schmidt et al., Prot. Engin. 11 (1998) 601-607) or Skp protein (Hayhurst and Harris, Protein Expr. Purif 15 (1999) 336-343).
The subject invention provides a process for the production of a naturally folded eukaryotic polypeptide containing at least two cysteines linked by disulfide bridges. The process comprises culturing in a nutrient medium prokaryotic cells which contain (i) an expression vector that encodes the polypeptide, and contains a prokaryotic signal sequence at the N-terminus, and (ii) an expression vector that encodes a molecular chaperone. The culturing is under conditions such that the polypeptide and the chaperone is secreted into the periplasm of the prokaryotic cells or into the medium.
The signal sequence is cleaved from the polypeptide and the polypeptide is isolated. Preferably, the signal sequence is derived from gram-negative bacteria.
Preferably, a reducing thiol reagent, such as glutathione, can also be added to the nutrient medium. Preferably, the molecular chaperone is a small heat shock protein (sHsp type) or a heat shock protein with a molecular mass of about 40 kDa (Hsp40 type). The nucleic acid coding for the polypeptide and the chaperone can be located on one vector or on two separate vectors. The DNA encoding the molecular chaperone preferably is in operative linkage with DNA encoding a signal peptide for penetrating the inner bacterial membrane.
The DNA encoding the secreted protein is preferably under the control of an inducible expression signal. While not limiting the choice of polypeptide, the polypeptide can be an antibody, antibody fragment, interferon, protein hormone, or a protease.